The number of angiosperm species for which nuclear DNA amount estimates have been made has nearly trebled since the last collected lists of such values were published, and therefore, publication of a more comprehensive list is overdue. This paper lists absolute nuclear DNA amounts for 753 angiosperm species. The data were assembled primarily for reference purposes, and so the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists whom, it is anticipated, will be among its major users. The paper also reviews aspects of the history, nomenclature, methods, accuracy and problems of nuclear DNA estimation in angiosperms. No attempt is made to reconsider those aspects of nuclear DNA estimation which have been fully revised previously, although the bibliography of such aspects is given. Instead, the paper is intended as a source of basic information regarding the terminology, practice and limitations of nuclear DNA estimation, especially by Feulgen microdensitometry, as currently practiced.

Nuclear DNA Amounts in Angiosperms

Reproductive development of plants is highly vulnerable to water deficit. Stress early during this phase can delay or completely inhibit flowering, both through an inhibition of floral induction and development. The stage of meiosis is perhaps the most stress-sensitive period of reproduction in all species studied. A meiotic-stage water deficit causes pollen sterility, but usually affects female fertility only under extreme stress. Sterility occurs even though the reproductive structures of stressed plants maintain a high water status, indicating that the response is probably mediated by a sporocidal signal from elsewhere in the plant. Rice and maize plants are also highly vulnerable during flowering (anthesis) and early grain initiation. Stress during this period can cause loss of pollen fertility, failure of pollination, spikelet death, or zygotic abortion. Changes in carbohydrate availability and metabolism appear to be involved in the effects of stress during meiosis and anthesis. Stress during early grain development curtails the kernel sink potential by reducing the number of endosperm cells and amyloplasts formed. Controls underlying these effects are poorly understood, although hormones may be involved. A water deficit during any stage of grain development causes the premature cessation of grain filling. Kernel moisture content and its direct impact on metabolism appear to be key regulatory factors in shortening the duration of grain filling.

Reproductive Development in Grain Crops during Drought

Male reproductive development in plants is highly sensitive to water deficit during meiosis in the microspore mother cells. Water deficit during this stage inhibits further development of microspores or pollen grains, causing male sterility. Female fertility, in contrast, is quite immune to stress. The injury is apparently not caused by desiccation of the reproductive tissue, but is an indirect consequence of water deficit in the vegetative organs, such as leaves. The mechanism underlying this stress response probably involves a long-distance signaling molecule, originating in the organs that undergo water loss, and affecting fertility in the reproductive tissue, which conserves its water status. Much research has been focused on the involvement of abscisic acid in this regard, but the most recent evidence tends to reject a role for this hormone in the induction of male sterility. Stress-induced arrest of male gametophyte development is preceded by disturbances in carbohydrate metabolism and distribution within anthers, and an inhibition of the key sugar-cleaving enzyme, acid invertase. Since invertase gene expression can be modulated by sugar concentration, it is possible that decreased sugar delivery to reproductive tissue upon inhibition of photosynthesis by stress is the signal that triggers metabolic lesions leading to failure of male gametophyte development.

Effects of water stress on male gametophyte development in plants

Transferring wheat (Triticum aestivum L cv Banks) from 21/16°C to high temperature (36/31°C) for intervals of 2 days in the period from head emergence to 10 days after anthesis resulted in grain sterility and specific forms of morphological and cellular damage depending on the stage of development of the grain at the time of transfer Grain sterility was induced by high temperature 2-3 days prior to anthesis and this response was found to be considerably enhanced by high humidity Parthenocarpic, abortive and shrunken grain were induced by high temperature between anthesis and 3 days after anthesis High temperature from 6 to 10 days after anthesis resulted in notched, split and opaque grains

The response to high temperature shock and humidity changes prior to and during the early stages of grain development in wheat.

Wheat plants (cv. Gabo) otherwise grown at 20°C were subjected to a temperature of 30°C for 3 days at the onset of meiosis in the anthers. Control plants were maintained at 20°C throughout development. Serial sections through the heat-stressed ovaries just prior to anthesis showed that a third contained abnormal embryo sacs. Abnormalities ranged from the complete absence of an embryo sac accompanied by reduced nucellus development, to small embryo sacs that contained the full complement of cells. No abnormalities were observed in control ovaries. Following pollination with fertile pollen, heat-stressed stigmas had similar numbers of germinated pollen grains to non-stressed controls but there were fewer tubes reaching the ovary. In 7% of the stressed pistils, no pollen tube reached the ovary. Callose was deposited in some of the inhibited pollen grains and tubes that showed abnormal growth. It is concluded that heat stress during meiosis in wheat can reduce yield by causing abnormal ovary development, which results in reduced pollen tube growth and seed set.

Effect of Heat Stress During Floral Development on Pollen Tube Growth and Ovary Anatomy in Wheat (Triticum aestivum L.)

The developmental behaviour of reproductive cells was studied during premeiotic mitotic activity, premeiotic interphase, meiosis in anthers and ovaries, microsporogenesis, megasporogenesis and embryo and endosperm growth in Triticum aestivum L. var. Chinese Spring. Particular attention was paid first to the timing and rate of cell development in the anthers and the ovary within a floret and secondly , to the timing and rate of nuclear and cell development in the young embryo and endosperm. At 20 °C the development studied lasted in each floret about 21 days starting 7 days prior to meiosis in anthers and ending 5 days after anther dehiscence and pollination. The durations of up to twenty successive cell cycles were estimated. In anthers of plants grown at 20 °C the durations of the three successive cell cycles immediately prior to the cycle which ends at first anaphase of meiosis were about 25, 35 and 55 h respectively. The increase in cell cycle time was correlated with an increase in the size of archesporial cells and their nuclei. A progressive increase in the durations of successive cell cycles as meiosis is approached has not been measured previously in a higher plant species, although it has been noted in the germ line cells of male mice. The pollen mother cells (p.m.cs) within an anther were synchronized prior to meiosis by having their development blocked somewhere in G 1 of premeiotic interphase. The developmental hold began to operate about 103 h prior to the synchronous onset of meiosis in all the p.m.cs within an anther at 20 °C. About 55 h later, when the last archesporial cell completed its final premeiotic mitosis, all the p.m.cs had accumulated in G 1 of premeiotic interphase and synchrony was complete. Premeiotic interphase after all the p.m.cs were first synchronized in G 1 until the synchronous onset of meiosis lasted about 48 h. During this period the G 1 developmental hold was released and p.m.cs initiated DNA synthesis synchronously about 12 to 15 h prior to the start of leptotene. Meiosis in p.m.cs lasted 24 h at 20 °C. Within each floret, meiosis in p.m.cs was almost or quite synchronous with, and had the same duration as, meiosis in the embryo sac mother cell. The Q 10 for meiosis in p.m.cs over the temperature range 15 to 25 °C was about 2.3. Microsporogenesis from tetrad stage until anther dehiscence lasted about 7.5 days at 20 °C. The first pollen grain mitosis (p.g.m. 1) occurred 2.5 days and second pollen grain mitosis (p.g.m. 2) 5.0 days after the end of meiosis. Concurrent with p.g.m. 1 the functional megaspore in the ovule of the same floret divided. This division was rapidly followed by two more synchronous division cycles (also concurrent with p.g.m. 1) which produced an 8-nucleate embryo sac. By p.g.m. 2 the embryo sac contained 20 to 30 antipodal cells at its chalazal end. The antipodal cells subsequently became highly polyploid and some eventually contained up to 200 times as much DNA as haploid egg nuclei. At 20 °C the sperm nuclei reached the egg and polar nuclei about 40 min after pollination. The primary endosperm nucleus divided about 6 h after pollination while the zygote did not divide until about 22 h after pollination. The endosperm often contained 16 mitotic nuclei 24 h after pollination. The nuclear division cycle during the first five division cycles was about 4.5 h. Until the tenth division cycle when the endosperm became a cellular tissue, development of endosperm nuclei was synchronous, but thereafter synchrony was progressively lost. Early embryo development was marked by a gradual decrease in the durations of successive cell cycles. This decrease was apparently correlated with a decrease in the size of embryo cells and their nuclei. Nuclear and cellular developmental rates at 20 °C were very variable. Estimates of nuclear cycle times ranged from about 60 h in the microspore to about 4.5 h in some endosperm nuclei. Nuclear volume was also very variable and ranged from about 240 μm 3 for sperm nuclei in mature microspores to about 160000 μm3 in some polyploid antipodal cells. Both the wide range of nuclear types described, and the speed with which nuclear characters changed, illustrates the remarkable plasticity of the wheat nucleus which may occur in several very different forms. A comprehensive and integrated study of development at the cellular level in reproductive tissues of a higher plant species is presented. The importance of this study is twofold. First, it allows the comparison of reproductive cell behaviour in a higher plant species and in those animal species which have been intensively examined. Secondly the availability of a description of ‘normal’ development under controlled conditions in euploid plants of Chinese Spring opens the way for comparative studies using the wide range of available mutant or chromosomally different genotypes of Chinese Spring which are known to vary in their reproductive development.

M. W. Bayliss

Papers

Cell development in the anther, the ovule, and the young seed of Triticum aestivum L. var. Chinese Spring